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Arterial supply to brain:Cerebellar arteries (SCA, AICA, PICA)
The superior cerebellar artery (SCA) arises from the distal basilar artery, just below the posterior cerebral artery (PCA) and typically supplies the superior half of the cerebellum, and the midbrain. * whole superior surface of the cerebellar hemispheres down to the great horizontal fissure * the superior vermis * dentate nucleus * most of the cerebellar white matter * parts of the midbrain (e.g. inferior colliculus by the quadrigeminal segment) Unlike the anterior inferior cerebellar artery (AICA) and posterior inferior cerebellar artery (PICA) the SCA is usually present but, it is frequently duplicated: * unilateral duplication: 28% * bilateral duplication: 8% * triplication: 2% The upper branch, when multiple, may arise from the PCA and usually supplies the vermis, whereas the lower branch supplies the hemisphere. Segments * prepontine segment: It passes lateralward, immediately below the oculomotor nerve, which separates it from the posterior cerebral artery. * ambient segment: winds around the cerebral peduncle, close to the trochlear nerve * quadrigeminal segment: continues on the upper surface of the cerebellum Branches * perforating branches ** pons ** midbrain ** inferior colliculus * lateral (marginal) branch ** largest branch of the SCA ** usually arises from the ambient segment ** runs posteriorly in the region of the interhemispheric fissure ** gives off hemispheric branches that course superiorly over the superior cerebellar hemisphere * hemispheric branches ** arise from the lateral (marginal) branch and from the SCA distal to the origin of the lateral branch also from the ambient segment ** supplies dentate nucleus ** superior vermis ** medial quadrigeminal lobule ** superior semilunar lobule * superior vermian ** terminal branch(es) of the SCA ** anastomose with inferior vermian branches of the PICA The anterior inferior cerebellar artery (AICA) is another vessels that provides arterial blood supply to the cerebellum. It has a variable origin, course and supply, with up to 40% of specimens not having an identifiable standard AICA due to PICA-AICA dominance. It usually includes: * middle cerebellar peduncle * infrolateral portion of the pons * flocculus * anteroinferior surface of the cerebellum 99% of AICAs arise from the basilar artery with around 75% coming from the lower third. It passes backwards to the anterior surface of the cerebellum. Branches * internal auditory branch (80% single, 20% double) passes into the internal acoustic meatus * lateral branch passes around the flocculus and into the hemispheric fissure (supplying both superior and inferior semilunar lobules) * medial branch supplies the biventral lobule Before cross-sectional imaging, the AICA (along with venous displacement) was used to identify posterior fossa intra- or extra-axial masses especially at the CP angle. Extra-axial masses (e.g. acoustic schwannomas or meningiomas) would displace the vessel whereas intra-axial masses tend not to. Occlusion of AICA results in lateral pontine syndrome, also known as AICA syndrome., which involve sudden onset vertigo and vomiting, nystagmus (due to damage to vestibular nuclei), ipsilateral loss of sensation of the face (due to damage to principal sensory trigeminal nucleus), ipsilateral facial paralysis (due to damage to the facial nucleus) and ipsilateral hearing loss and tinnitus (due to damage to the cochlear nuclei). Posterior inferior cerebellar artery (PICA) is one of the three vessels that provide arterial supply to the cerebellum. It is the most variable cerebral artery. Its origin is usually the vertebral arteries intracranially but variations include: * 18-20% arise extracranially, inferior to the foramen magnum. * 10% arise from the basilar rather than vertebral artery * 2% bilaterally absent * occasionally loops around the cerebellar tonsil Segments Branches from this artery supply the choroid plexus of the fourth ventricle. * anterior medullary segment ** from its origin around the inferior aspect of the medullary olive * lateral medullary segment ** curves forming the 'caudal loop' which is located anteroinferior to the tip of the cerebellar tonsil between the origins of the vagus and the accessory nerve. ** the apex of the loop is *** above foramen magnum in 60% of cases *** at the level of FM in 10% *** and below in 30% * posterior medullary segment ** ascends posterior to the medulla behind CN IX and CN X and along the posterior medullary velum. * supratonsillar segment ** junction between the posterior medullary segment and the supratonsillar segment is upwardly convex and is the site of origin of small choroidal branches: it is known as the "choroid point". This point has a constant relationship with the 4th ventricle and was used prior to crossectional imaging to asses for shift in its position. Branches * anterior and lateral medullary segments ** small perforating medullary branches (absent in 50%) * supratonsillar segment ** tonsillohemispheric branch ** inferior vermian branch Supply Has a variable territory depending on the size of the AICA. Typically it may supply: * posteroinferior cerebellar hemispheres ** cerebellar tonsils: 85% of the time ** biventral lobule: 80% ** neucleus gracilis: 85% ** superior semilunar lobule: 50% * inferior portion of the vermis * lower part of the medulla: 50%